Welcome
I assume you did an import from your previous program into Gramps. How did your previous application handle Sources and Citations?
Even if you could now do a bulk import of just Source information, that is all you would have, a list of sources not attached to any person or event.
When I migrated my database to Gramps, I had a database that was not adequately sourced. So now I am going family by family, person by person, adding the source/citations records. In the process, I am able to add more information about these people and families.
And yes, it is a slow but rewarding process.